1. Field of the Invention
The present invention is directed to an automated DNA sequencer or forensics device that holds an electrophoretic gel.
2. Related Art
Electrophoresis devices process samples containing DNA and/or protein fragments in carrier media, such as separation gels, for reading a sequence of a particular sample by, for example, optical detection methods. In a typical optical detection technique, fluorescence-labeled DNA fragments are migrated in gel slabs and then excited with a light source at one wavelength to produce a fluorescence signal at another wavelength that corresponds to a particular DNA fragment.
Electrophoresis has conventionally been performed using a "gel sandwich" which comprises two glass plates separated by a small gap distance. The gap is used to hold an agarose or acrylamide/polyacrylamide gel in which the electrophoretic separation occurs. The gap distance defines the thickness of the gel lanes of the gel sandwich. For example, the LTI S2 manual sequencer, distributed by Life Technologies Inc., of Rockville Md., utilizes gel lanes on the order of 0.016 inches (or about 0.4 millimeters (mm)) thick, and approximately 0.2 inches in width. Smaller gel plate gap distances are generally preferred in apparatuses utilizing small gel pore sizes, with preferable gap distances being on the order of 0.1 mm to 3 mm. See, e.g., U.S. Pat. No. 5,529,679 issued to Takahashi et al. ("Takahashi"). Larger gap distances are less desired because they require higher operational voltages to be applied to the electrophoretic gel, resulting in greater heating. Other conventional sequencers include capillary devices, such as that described in the Takahashi patent, and devices that utilize thin, ribbed plastic sheets to hold the electrophoretic gel, such as that described in U.S. Pat. No. 4,374,723 to Vesterberg.
The operation of a conventional gel sandwich takes place as follows. DNA fragments are placed in "wells" formed at the top end of the gel sandwich. These wells are typically formed by the use of a comb that defines the width and number of gel lanes. A voltage is then applied across the gel. The voltage causes particle separation of the sample to occur as a function of the pH and pore size of the gel, the cross section of the gel, the molecular size and charge of the sample, and the applied voltage.
A laser/reader can be configured at the bottom of the standard sequencer gel sandwich to scan back and forth to detect fluorescence. As the DNA bands develop in the electrophoresis process and travel past the detection point, the laser excites individual bands and a sensor reads the bands by scanning the fluorescence as each band passes the reading point.